The present invention relates to a speed control system for automotive vehicles, and more particularly to an improvement of an electronic speed control system for vehicles of the type in which the opening angle of the engine throttle is controlled in accordance with the actual speed of the vehicle and the rate of change of the actual speed in relation to a desired command speed.
For instance, in such a speed control system as disclosed in U.S. Pat. No. 3,893,954 issued on Oct. 5, 1976, voltages dependent on vehicle speed and rate of change of speed are compared with a reference voltage representing a desired speed and the resultant output is compared with a periodic signal to produce output pulses of constant frequency and of a width varying with the relative magnitudes of the input voltages. These pulses are applied to an electromagnetic valve which controls the admission of pulses of reduced gas pressure from the vehicle engine inlet manifold to a chamber of variable volume and the resultant movement of a wall of the chamber is employed to control the engine throttle. This means that operation of the electromagnetic valve is frequently conducted in response to the output pulses to maintain the vehicle speed in a predetermined value. As a result, it will be experienced that the life of electromagnetic valve is shortened due to its frequent operation.